Mac startup problem

Hi all, sometimes when i start up my computer the whole screen looks like a plaid shirt. it looks to me like the whole screen has been ghosted on in several different places across the screen. Say the apple icon is in the middle of the screen. I can see bits and pieces of it all over the screen. and then the whole screen will turn dark gray starting from the top and i will be asked to restart the computer. This message is also ghosted around the screen in bits and pieces. after turning off my computer and restarting mabe 30 times i might be able to get a normal startup where i have then discovered that it can only run stable (not ghost everything everywhere) if i play a dvd. weird right? ive tried holding down the power button until it beeps. this does not help. Ive also started up in the command prompt where you hold down Command S on startup, sometimes it helps if i check the disk in this mode but it always says it's ok but it will sometimes fix the problem somehow, but only for a second as its checking the disk. I seriously need some help because i don't want to have to play a dvd in the background just to use my computer.

Are you in Scotland?
(sorry)

Put in your OS installation disc and boot while holding down the C key. Then go into Disk Utility and repair permissions. Report your results here. Maybe there are other things to try.

Identify your Mac model, processor, OS, any relevant specs...